Cosmic ray transport in galaxy clusters: implications for radio halos, gamma-ray signatures, and cool core heating

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany
e-mail: ensslin@mpa-garching.mpg.de
² Canadian Institute for Theoretical Astrophysics, 60 St. George Street Toronto, Ontario, M5S 3H8, Canada
e-mail: pfrommer@cita.utoronto.ca
³ ETH Zurich Institute of Astronomy, Physics Department, HIT J 12.2. Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁴ Inter-University Centre for Astronomy & Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India

Received: 27 August 2010
Accepted: 22 December 2010

Abstract

We investigate the interplay of cosmic ray (CR) propagation and advection in galaxy clusters. Propagation in form of CR diffusion and streaming tends to drive the CR radial profiles towards being flat, with equal CR number density everywhere. Advection of CR by the turbulent gas motions tends to produce centrally enhanced profiles. We assume that the CR streaming velocity is of the order of the sound velocity. This is motivated by plasma physical arguments. The CR streaming is then usually larger than typical advection velocities and becomes comparable or lower than this only for periods with trans- and super-sonic cluster turbulence. As a consequence a bimodality of the CR spatial distribution results. Strongly turbulent, merging clusters should have a more centrally concentrated CR energy density profile with respect to relaxed ones with very subsonic turbulence. This translates into a bimodality of the expected diffuse radio and gamma-ray emission of clusters, since more centrally concentrated CR will find higher target densities for hadronic CR proton interactions, higher plasma wave energy densities for CR electron and proton re-acceleration, and stronger magnetic fields. Thus, the observed bimodality of cluster radio halos appears to be a natural consequence of the interplay of CR transport processes, independent of the model of radio halo formation, be it hadronic interactions of CR protons or re-acceleration of low-energy CR electrons. Energy dependence of the CR propagation should lead to spectral steepening of dying radio halos. Furthermore, we show that the interplay of CR diffusion with advection implies first order CR re-acceleration in the pressure-stratified atmospheres of galaxy clusters. Finally, we argue that CR streaming could be important in turbulent cool cores of galaxy clusters since it heats preferentially the central gas with highest cooling rate.